The present invention relates to a change-speed gearbox comprising an input shaft and an output shaft arranged coaxially, a layshaft parallel to the input shaft and the output shaft, a constant mesh gear formed by a gear stage and forming a drive connection between the input shaft and the layshaft, and a plurality of gear stages which form drive connections between the layshaft and the output shaft, the plurality of gear stages being optionally selectable and in conjunction with the constant mesh gear form non-geometrically graduated gear ratios.
A gearbox series including a change-speed gearbox of the general type described above designed as a six speed single-range gearbox S6-66 is shown in Publication F 43 542/RT 3427-789 from Zahnradfabrik Friedrichshafen AG, which can be expanded to a twelve-speed multi-speed gearbox S6-66+GV 66 by a splitter group GV 66 to form a gearbox series.
In gearbox series of this kind there is the problem that, for reasons of driving power, the single range gearbox is as a rule graduated progressively to give a favorable ratio jump between the two highest gears, i.e. in the known single-range gearbox between the gear ratios of the fifth gear and of the sixth gear. The result is that "splitting" of the main driving gears IV, V and VI gives rise to different split jumps. In order to avoid this disadvantage, the ratios of those gear stages in the known twelve-speed multi-speed gearbox S6-66+GV 66 which correspond to the gear stages for the formation of gears IV and V in the six-speed single-range gearbox S6-66 are modified relative to the relevant gear stages of the single-range gearbox S6-66. This makes it impossible, in the case of the known gearbox series, to use the same basic gearbox for both the single-range gearbox and the multi-speed gearbox elements.
German Patent Document DE 41 29 290 CI discloses a change-speed gearbox for motor vehicles in which an input shaft and an output shaft are arranged parallel to a layshaft and a respective drive connection can be established between the input shaft and the layshaft via two alternately selectable gear stages and a drive connection between the layshaft and the output shaft can be established via a further gear stage, and in which the layshaft gearwheel of one of the gear stages has the smaller number of teeth relative to the main shaft gearwheel meshing with it, and only two of the three gear stages have the same total number of teeth. In order to create a gearbox series which comprises a number of different types of gearbox for different permissible input torques while taking account of the requirement for the reduction of the number of different components to a minimum, this known change-speed gearbox is designed in such a way that a second layshaft gearwheel, which belongs to one of the two gear stages having the same total number of teeth, has the smaller number of teeth.
An object of the present invention is to provide a gearbox series in which it is possible to use an essentially unmodified basic gearbox for the single-range gearbox and the multi-speed gearbox.
This and other objects are achieved by the present invention which provides a change-speed gearbox comprising an input shaft and an output shaft arranged coaxially, a layshaft parallel to the input shaft and the output shaft, a constant mesh gear formed by a gear stage and forming a drive connection between the input shaft and the layshaft, and a plurality of gear stages which form drive connections between the layshaft and the output shaft, the plurality of gear stages being optionally selectable and in conjunction with the constant mesh gear form non-geometrically graduated gear ratios. The change-speed gearbox is configurable as a single-range gearbox and as a multi-speed gearbox, with identical gear stages used as drive connections between the layshaft and the output shaft for both the single-range gearbox and for the multi-speed gearbox configurations. The constant mesh gear of the single-range gearbox configuration has a smaller ratio than the constant mesh gear of the multi-speed gearbox configuration, the multi-speed gearbox configuration including a splitter group formed by a gear stage which can be selected alternatively with the constant mesh gear of the multi-speed gearbox to serve as the drive connection between the input shaft and the layshaft. In the multi-speed gearbox configuration, the quotient of the transmission ratio of the constant mesh gear and the transmission ratio of the splitter group is at least approximately equal to the square root of a ratio jump between the two highest gears without the splitter group.
When the change-speed gearbox according to the invention is used as a multi-speed gearbox, close, uniform gradation of the upper six driving gears is achieved and, despite this, only the constant mesh gear is modified as compared with the embodiment as a single range gearbox, reducing the number of different components required for the gearbox series to a minimum.
While, in the known twelve-speed multi-speed gearbox, the spread is greater only by the split jump than in the corresponding six-speed single-range gearbox, the spread in the twelve-speed multi-speed gearbox according to the invention is additionally increased, relative to the spread of the associated single-range gearbox, by the larger ratio of the constant mesh gear. This greater spread is particularly advantageous in multi-speed gearboxes because they are intended for heavier-duty applications than single-range gearboxes.